A total of 33 young boys (mean age 3.4 years) with Duchenne muscular dystrophy and 21 normal controls (mean age 3.5 years) were assessed using the Griffiths's mental development scales and the Reynell language scales. The boys with Duchenne muscular dystrophy were significantly developmentally delayed when compared with the control group. The developmental delay was most pronounced in locomotor function and language. There was no significant difference in social class distribution. Early diagnosis of Duchenne muscular dystrophy is of vital importance if secondary cases within families are to be prevented. While diagnosis is still unacceptably late in most cases, it can be improved if all boys with this pattern of developmental delay are screened for Duchenne muscular dystrophy by measurement of creatine kinase activity.